Organic Chemistry I
Organic Chemistry I CHM 2210C
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Date Created: 10/29/15
7 Alkenes Reactions and Synthesis Based on McMurry s Organic Chemistry 61h edition 71 Preparation of Alkenes A Preview of Elimination Reactions I Dehydrohalogenation removing HX from an alkyl halide H CC X gt CC HX I Elimination Reaction requires a strong base H Br H mm 7 cuucngmi 3 20 H H H clohexane Cyclohexene Bl ki B Preparation of Alkenes A Preview of Elimination Reactions I Dehydration removing water from an alcohol CH3 HSO HO CH3 OH 2 4i 2 O THF 50 C H2O Elimination Reaction requires a strong acid 0 THF tetrahydrofuran Problem Write the structures of all the possible alkenes capable of being produced when 2Bromo23 dimethylbutane undergoes dehydrohalogenation 704 vc cv X OH Alcohol Alkane HO OH C C C C Halohydrin 12Diol x X Ci c o 12Diha1ide 5233 H X Alkene C Cic C C Halide Cyclopropane 2004 39 72 Addition of Halogens to Alkenes gt C Clt X2 cl cl vicinal dihalide vicinal from the latin vicinalis neighboring an electrophilic addition electrophilic occurs easily with CI2 and BrZ occurs too easily with F2 does not react with 2 Complete the reaction ON 2 Br2 gt ch130 c Bromonium Ion Br Cyclopentene K Bromonium ion intermediate Q Br trans12 Dibrom ocyclopentane Lop sxae open Lo attack Q a Wquot H H H H B H Brigl gt x 1 Bottom side shielded from attack trans12Dibromo Bromonjum ion cyclopentane Cyclopentene intermediate 5 9 en 6 we 1 f 0 2111 h Bromonium ion Bromonium Ion I trans isomers suggest anti stereochemistry as opposed to syn stereochemistry I What type of isomers would suggest syn Practice I What product would you expect to obtain from the addition of Cl2 to 12 dimethylcyclohexene Show the stereochemistry of the product I The addition of HCI to 12 dimethylcyclohexene yields a mixture of two products Why 73 Halohydrin Formation C C E 1 X H O C C HX 2 2 l l halohydrin XC2 or Br2 Note Br2 is usually added indirectly using NBromosuccinimide NBS in aqueous dimethyl sulfoxide DMSO 0 QN Br NBS O Halohydrin Formation I Anti stereochemistry Q gt 7 H20 39 Halonium ion mechanism Q Br Br dz Bromonium ion J quot Equot Cyclopenlene H2OCH3SOCH3 DMSO 74 Hydration of Alkenes Oxymercuration I Hydration of an alkene is the addition of HOH to to give an alcohol I Acid catalysts are used in high temperature industrial processes ethylene is converted to ethanol In the presence of water and a strong acid OH H CHaC tliH HA CHQ39I Oxymercuration I An alternative method is oxymercuration I analogous to halohydrin formation I electrophilic addition of the mercury ion to the alkene to yield the mercurinium ion intermediate Mercuric acetate Hg02CCH32 or abbreviated HgOAc2 in aqueous tetrahydrofuran THF solvent Oxymercuration I the addition of sodium borohydride completes the reaction I the regiochemistry corresponds to Markovnikov s addition of water The hydroxyl group attaches to the most highly substituted carbon atom Oxymercuration 1 H OAC H OTHF 043 g 2 2 2 NaBH4 Oxymercuration Q IOAC HgOAc g OAc Hi5 CH3 CH3 gt gt H20THF CHa R 1Meihyicyclopeniene OAC SQAC mercurinium ion HgOAc EgtltCHa NaBHA gem 1Meihylcyclopenianol OAC organomercury intermediate Practice What product would you expect from the oxymercuration of these alkenes 1Pentene 2MethyI2pentene Practice What alkenes might these alcohols have been prepared from OH I CH3CIDCHzCHzCHzCH3 C 3 OH 75 Addition of Water to Alkcncs Hydroboration I Herbert Brown HB invented hydroboration HB I Borane BH3 is electron de cient is a Lewis acid I Borane adds to an alkene to give an organoborane H H BH B H 00 gt C C H g f An organoborane BH3 Is a Lewis Acid I Six electrons in outer shell I Coordinates to oxygen electron pairs in ethers Clottl39opliilic b H quot 39 i H a HanoO a HABiOO H C H Borane THF BH3 THF complex Thomson Brooks Cole HydroborationOxidation Forms an Alcohol from an Alkene I Addition of HBH2 from BH3THF complex to three alkenes gives a trialkylborane I Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene OH THF i304 e gig U Cyclohexene Cyclohexanol Tricyclohexylborane Thomson Brooks Cole Orientation in Hydration Via Hydroboration I Regiochemistry is opposite to Markovnikov orientation I OH is added to carbon with most H s I H and OH add with syn stereochemistry to the same face of the alkene opposite of anti addition 1 B H OH BIL OH CH3 W 6TH 6TH CH3 CH3 lMeth lo 010 entene y y P Alkylborane trans 2 Methsyggyclopentanol intermediate U Thomson Brooks Cole Mechanism of Hydroboration l Borane is a Lewis acid I Alkene is Lewis base l Transition state involves anionic development on B l The components of BH3 are across 00 in ll I ll u m I Hydroboration Orientation in Addition Step I Addition in least crowded orientation syn I Addition also is via most stable carbocation less Q crowdcd H 39 CH a 39 1 H uld H91 5 E H B H J B H CH3 H H H H Gives more stable 130 ullwm carbocation Hydroboration Electronic Effects Give NonMarkovnikov I More stable carbocation is also consistent with steric preferences I Iquot Parlial 3 union Inmro stable lrzlnsiliun stale I H Hgt CH HI asquot I Mcll39lc3 clupenlen n 1 39 quotg H uHii Partial 239 Elli l i H IV I39IIII39G less stable transition slate No steri c rowding more ka 39 5gt H J 2004 Thomson 7 BrooksCole Steric crowding here Hydroboration Oxygen Insertion Step I H202 OH39 inserts OH in place of B I Retains syn orientation R F1202 R gt H H 39 CH 3 H CH3 OH H B R R CHZCHS CHQCHg 1 131713 THF lt 1 HE QACIZ H20 CHZCH ltjH H m 77011 OH H H H a Syn nonMarkovnikov b Markovnikov addition of 1 120 addition of H20 2004 Thomson A BrooksCole Practice What product results from the hydroborationoxidation of lmethylcyclopentene with deuterated borane BD3 Show both the stereochemistry spatial arrangement and the regiochemistry orientation of the products Practice What alkenes might be used to prepare the following alcohols by hydroboration oxidation 76 Addition of Carbenes to Alkenes I The carbene functional group is half of an alkene I Carbenes are electrically neutral with six electrons in the outer shell I They behave as electrophiles I Form cyclopropanes R39 W K A C 200 R quotI M gl lggpe A carbene A cvclonnmane Formation of Dichlorocarbene I Base removes proton I i form from ch oro Ll H I Stabilized carbanion Cl ains A k 39ll I Unimolecular v Elimination of Cl gives electron deficient iirr In species I dwhlorocarbene riclllurumcklmnidu Iniim Cl C 39r C C4 C1 H Strong base abstracts the chloroform CI proton leaving behind the electron 6 pair from the C H bond and forming the trichloromethanjde anion C Cl Cl H30 Cl Trichloromethanide anion Loss of a chloride ion and associated electrons from the CiCl bonds yields 1 the neutral dichlorocarbene Cl C C1 C1 4mm 9 n mul 39 ml 1 mm A Low W l 3 r munuullm 4 tin RR L e 9 ClC RC R a L a 39 77 wind L Dichlorocarbcnc A carbocation spghybridized u my masonBlanks Cola Reactions of dichlorocarbene The reaction of dichlorocarbene with alkenes is stereospeci c Starting with a cisdisubstituted alkene Only a cisdisubstituted cyclopropane is produced see next slide C1 931 H I39OH CC CHCL3 gt KC CH3CH2 CH3 xci CH CH3CH2 3 cis2Pentene H I pi CHCL3 I OH KCl Cl H Cyclohexene 2004 Thomson v BraoksCoie Simmons Smith Reaction I Equivalent of addition of CH2 I Reaction of diiodomethane with zinccopper alloy produces a carbenoid species I Forms cyclopropanes by cycloaddition see next slide CHZIZ ZnCu M I CHZ Zn I 2CH2 Diiodomethane Iodomethylzinc iodide a carbenoid Zm Cu CH212 Ether Cyclohexene Bicyclo410heptane 92 2004 Thomson 7 BrooksCole 11 Reactions of Alkyl Halides Nucleophilic Substitutions and Eliminations Part 1 Alkyl Halides React with Nucleophiles and Bases I Alkyl halides are polarized at the carbonhalide bond making the carbon electrophilic I Nucleophiles will replace substitute the halide in C X bonds of many alkyl halidesreaction as Lewis base I Nucleophiles that are Bronsted bases produce elimination r inl l inlh liluliun Vliquot39l ifix Ni39 m H Elimination mic 4 Cc l min Xv 39 Tmmguri mm Chapter 1 1 Reactions I SN2 I Substitution Nucleophilic Second Order I SN1 I Substitution Nucleophilic First Order I E2 I EHnnna on Second Order I E1 I Elimination First Order The Nature of Substitution I Substitution by definition requires that a quotleaving groupquot which is also a Lewis base departs from the reacting molecule I A nucleophile is a reactant that can be expected to participate effectively in a substitution reaction Y gt Y9 nucleophile Leaving group Substitution Mechanisms I 3N1 I Two steps with carbocation intermediate I Occurs in 3 allyl benzyl I 3N2 I Two steps combine without intermediate I Occurs in primary secondary Two Stereochemical Modes of Substitution I Substitution with inversion A 3 Xe A B e C gt C Y 4 b I a Y H H X I Substitution with retention 9 A x A 728 gtch e Y H X 9H 114 The 8N2 Reaction I Reaction is with inversion at reacting center I Follows Second Order reaction kinetics I Ingold nomenclature to describe characteristic step I Ssubstitution I N subscript nucleophilic I 2 both nucleophile and substrate in characteristic step second order bimolecular 8N2 Process I The reaction involves a transition state in which both reactants are together CH H3C I 3 f39c Br gt HO C Br HO H CHZCHa H CHZCH3 8N2 Transition State I The transition state of an 8N2 reaction has a planar arrangement of the carbon atom and the remaining three groups Consider the following 8N2 rxn CHsBr OH39 gt CH30H Br I The rate determining step is bimolecular I Rate kCHsBrOH I The reaction occurs in a single step I Both methyl bromide and the hydroxide ion are involved in the transition state I The nucleophile OH pushes off the leaving group Br from its point of attachment to the carbon atom direct displacement Practice I Problem What product would you expect from the following reaction predict its stereochemistry and sign of rotation H CH3CH25 OH Br CH3 S2Bromooctane Write the rate law forthe reaction 115 Characteristics of the 8N2 Reaction I The rates of 8N2 reactions depend upon the following four variables I Substrate I Attacking Nucleophile I Leaving Agent I Solvent The Substrate I Sensitive to steric effects I Methyl halides are most reactive I Primary are next most reactive I Secondary might react I Tertiary are unreactive by this path I No reaction at 00 vinyl halides Steric Effects Reactivity of Some Alkyl Bromides in SN2 Reactions Alkyl Structure Class Relative Bromide Rate Methyl CH3Br unsubstituted 221000 bromide Ethyl CH3CH2Br primary 1350 bromide lsopropyl CH32CHBr secondary 1 bromide tert Butyl CH33CBr tertiary ltlt1 bromide SteIic Effects on SNZ Reactions m an H Iquot H SteIic Effects Steric Hindrance Raises Transition State Energy Very hindered 1min Lnliinderetl substqu 39 Rciiitliurl progress I Steric effects destabilize transition states I Severe steric effects can also destabilize ground state 8N2 reactions do not occur with aryl halides nor vinylic halides R Cl V n 6 No reaction C1 ltgt N0 reaction 39 Aryl halide N u Vinylic halide 2004 Thomson 7 BrooksCole Practice I Identify the compound of the following pairs that reacts with sodium iodide in acetone at the faster rate I 1Chlorohexane or cyclohexyl chloride I 1Bromopentane or 3bromopentane The Nucleophile I Neutral or negatively charged Lewis base I Reaction increases coordination at nucleophile I Neutral nucleophile acquires positive charge I Anionic nucleophile becomes neutral I See Table 111 for an illustrative list A xNL uLml Negatively charged Nut R Y gt R Nu Y Nui F 0 7 Neutral Nu Nut R Y gt R NLI Yl Positively Chnrged Thomson Brooks Cole Relative Reactivity of Nucleophiles I Depends on reaction and conditions I More basic nucleophiles react faster for similar structures See Table 112 I Better nucleophiles are lower in a column of the periodic table I Anions are usually more reactive than neutrals CHuiBi39 liH 4 CHgiNn Biquot Nu ll l Jl L39ll Nll l39l39 llll fill J N7 HSquot f39 quot quot l 31m 74m mun mum 130410 lUUJJlH 121W LZEW irnrlwiw Mon mmve r97 Tnomsan Brooks Cole Relative Reactivity of Nucleophiles I Neutral Lewis bases such as water alcohols and carboxylic acids are much weaker nucleophiles than their conjugate bases ROis more nucleophilic than RZOH alkoxide ion alcohol Nucleophilicity of Some Common Nucleophiles Reactivity class Nucleophile Relative reactivity Very good I HS RS gt105 nucleophiles Good Br HO RO 104 nucleophiles CN N3 Fair nucleophiles NH3 CI F 103 R002 Weak H20 ROH 1 nucleophiles Very weak RCO2H 10 2 nucleophiles Practice lWhat products would you expect from the reaction of 1bromobutane with these reagents I Nal I KOH I NH3 H CEC Li Practice I Which of the following pairs of reagents are more nucleophilic I CH32N or CH32NH I CH33B or CH33N I H20 or H28 The Leaving Group I A good leaving group reduces the barrier to a reaction I Stable anions that are weak bases are usually excellent leaving groups and can delocalize charge Leaving mm 7 l ALCiY u c Y a mic Yr U it 39l39runxiliun slate Negative charge is delocalized overhaul Nu and Y OHquot NHZ OR F 01 B1 139 TDsO39 gv 1 Relative 39eactivity ltlt1 l 200 10000 30000 6000 Less More reactive reactive 2004 Thamson r BrooksCole Poor Leaving Groups I Ifa group is very basic or very small it is prevents reaction RF R OH R OR R NH2 These compounds do not undergo 5x2 reactione mum 1hnrvam n The Solvent I Solvents that can donate hydrogen bonds OH or NH slow 8N2 reactions by associating with reactants I Energy is required to break interactions between reactant and solvent I Polar aprotic solvents no NH OH SH form weaker interactions with substrate and permit faster reaction I acetonitrile CH3CN I DMSO CH3ZSO I and HMPA CH32N3PO CH3CHZCH20H27B1 N 8 CHSCHZCHZCHgiNH Br Solvent CH 30H H 20 DMSO DMF CHSCN HMPA Relative 1 7 1300 2800 5000 200000 eact1v1ty Less More reactive 39 reactive 2004 rhomson r BrooksCole f A solvated anion R0 H X5 vH OR reduced nucleophilicity due to 5 enhanced groundstate stability 2004 Thomson 7 BrooksCole 8N2 in a nutshell I Substrate Steric hindrance raises the energy of the transition state thus increasing AG1 and decreasing the reaction rate As a result 8N2 reactions are best for methyl and primary substrates I Nucleophile More reactive nucleophiles are higher in energy thereby decreasing AG1 and increasing the reaction rate 8N2 in a nutshell I Leaving Group Good leaving groups more stable anions lower the energy of the transition state thus decreasing AG1 and increasing the reaction rate I Solvent Protic solvents form hydrogen bonds to the nucleophile thereby lowering the energy of the nucleophile increasing AG1 and decreasing the reaction rate Polar aprotic solvents surround the accompanying cation but not the nucleophilic anion thereby raising the energy of the nucleophile decreasing AG1 and increasing the reaction rate 39llinrlewrl Substrate 39n hi n lererl substrate Reaction progress a Guntl nucleuphilo 39 Pum39 nucleuphilu Al nur len n I39nnp Goad leaving group Reaction progress v b Reaction progress 0 mm MarxismEmma om Polar aprotic solvent Prolic sulrcm Reaction progress 1 End of Chapter 11 part 1